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On continuous extension of uniformly continuous functions and metrics

100%

Banakh T. , Brodskiy N. , Stasyuk I. , Tymchatyn E.

2009

tom 116

nr 2

191-202

We prove that there exists a continuous regular, positive homogeneous extension operator for the family of all uniformly continuous bounded real-valued functions whose domains are closed subsets of a bounded metric space (X,d). In particular, this operator preserves Lipschitz functions. A similar result is obtained for partial metrics and ultrametrics.

Coarse structures and group actions

100%

Brodskiy N. , Dydak J. , Mitra A.

nr 1

149-158

The main results of the paper are: Proposition 0.1. A group G acting coarsely on a coarse space (X,𝓒) induces a coarse equivalence g ↦ g·x₀ from G to X for any x₀ ∈ X. Theorem 0.2. Two coarse structures 𝓒₁ and 𝓒₂ on the same set X are equivalent if the following conditions are satisfied: (1) Bounded sets in 𝓒₁ are identical with bounded sets in 𝓒₂. (2) There is a coarse action ϕ₁ of a group G₁ on (X,𝓒₁) and a coarse action ϕ₂ of a group G₂ on (X,𝓒₂) such that ϕ₁ commutes with ϕ₂. They generalize the following two basic results of coarse geometry: Proposition 0.3 (Shvarts-Milnor lemma [5, Theorem 1.18]). A group G acting properly and cocompactly via isometries on a length space X is finitely generated and induces a quasi-isometry equivalence g ↦ g·x₀ from G to X for any x₀ ∈ X. Theorem 0.4 (Gromov [4, p. 6]). Two finitely generated groups G and H are quasi-isometric if and only if there is a locally compact space X admitting proper and cocompact actions of both G and H that commute.